General ultrasonic diagnostic apparatuses can acquire information inside a living organism by transmitting ultrasonic waves and receiving the ultrasonic waves reflected inside the living organism. This allows a diseased part, such as cancer, to be detected. Furthermore, imaging of physiological information, that is, functional information, of a living organism attracts attention to improve the detection efficiency. Photoacoustic tomography (PAT) that uses light and ultrasonic waves has been proposed as means for imaging functional information.
The photoacoustic tomography is a technology for imaging internal tissue, which serves as the source of acoustic waves, using the photoacoustic effect in which acoustic waves (typically ultrasonic waves) are generated by applying pulsed light generated from a light source to a subject and absorbing the light that has propagated and diffused in the subject. Changes in the received acoustic waves with time are detected at a plurality of locations, and the acquired signals are mathematically analyzed, that is, reconstructed, and information concerning optical characteristic values of the internal part of the subject is visualized in three dimensions.
The resolution of a three-dimensional image obtained using the photoacoustic tomography depends on the following factors, depending on the placement of acoustic detection elements. If a plurality of acoustic detection elements are placed on a planar surface, a resolution in a direction parallel to the placement planar surface (lateral resolution) depends on both the sizes of the receiving portions of the individual acoustic detection elements and frequencies that the acoustic detection elements can detect, and a resolution in a direction perpendicular to the placement planar surface (depth resolution) depends only on frequencies that the acoustic detection elements can detect. The resolution in the direction perpendicular to the placement planar surface is higher than the resolution in the parallel direction because it is generally easier to increase the frequencies that can be detected by the acoustic detection elements than decrease the size of the receiving portions. In the case where a plurality of acoustic detection elements are placed on a spherical surface, information in the depthwise direction of all of the acoustic detection elements are superimposed, and thus, the lateral resolution is also equal to the depth resolution. That is, since the resolution in all directions depends only to the frequencies, this placement offers high resolution. With intermediate placement between planar placement and spherical placement in which a plurality of acoustic detection elements are placed on a plurality of planar surfaces provided at different angles, the resolution less depends on the sizes of the receiving portions of the acoustic detection elements as the placement approaches from the planar placement to the spherical placement, thus allowing higher resolution to be achieved.
An example of an apparatus in which a plurality of acoustic detection elements are placed on a spherical surface is disclosed in PTL 1. In PTL 1, acoustic detection elements are placed in a spiral pattern on a hemispherical surface, and light irradiation and reception of acoustic waves using the acoustic detection elements are performed while the hemisphere is being rotated about a line connecting the poles of the hemisphere and the center of the sphere. Image reconstruction is performed to obtain image data by using signals output from the acoustic detection elements that have received the acoustic waves.